Asthma is a complex disorder. Both hereditary and environmental factors—allergies, viral infections, irritants—are involved in the onset of asthma and in its inflammatory exacerbations. More than half of asthmatics (adults and children) have allergies; indeed, allergy to house dust mite feces is a major factor in the development of the disease and in the occurrence of exacerbations. Infection with respiratory syncytial virus during infancy is also highly associated with the development of asthma, and viral respiratory infections often trigger acute episodes.
The introduction three decades ago of bronchodilating β2-agonists—adrenergic agonists selective for the β2 receptor—revolutionized the treatment of asthma. These agents proved to be more potent and longer acting (4-6 hours) than the nonselective adrenergic receptor agonists such as isoproterenol, which stimulate both α- and β-adrenergic receptors. β2-agonists give rapid symptomatic relief and also protect against acute bronchoconstriction caused by stimili such as exercise or the inhalation of frigid air. Frequency of use can also serve as an indicator of asthma control. Recently, an extra long-acting β2-agonist-salmeterol (duration up to 12 hours) was introduced in the United States. Salmeterol is so potent that it may mask inflammatory signs; therefore, it should be used with an anti-inflammatory.
Theophylline is a relatively weak bronchodilator with a narrow therapeutic margin (blood level monitoring is recommended to avoid toxicity) and a propensity for drug interactions (competition for hepatic cytochrome P450 drug-metabolizing enzymes alters plasma levels of several important drugs metabolized by that same system).
Moderate asthma is treated with a daily inhaled anti-inflammatory-corticosteroid or mast cell inhibitor (cromolyn sodium or nedocromil) plus an inhaled β2-agonist as needed (3-4 times per day) to relieve breakthrough symptoms or allergen- or exercise-induced asthma. Cromolyn sodium and nedocromil block bronchospasm and inflammation, but are usually effective only for asthma that is associated with allergens or exercise and then, typically, only for juvenile asthmatics. Inhaled corticosteroids improve inflammation, airways hyperreactivity, and obstruction, and reduce the number of acute exacerbations. However, it takes a month before effects are apparent and up to a year for marked improvement to occur. The most frequent side effects are hoarseness and oral candidiasis. More serious side effects have been reported—partial adrenal suppression, growth inhibition, and reduced bone formation—but only with the use of higher doses. Beclomethasone, triamcinolone, and flunisolide probably have a similar mg-for-mg potency; the newer approvals budesonide and fluticasone are more potent and reportedly have fewer systemic side effects.
Even patients with mild disease show airways inflammation, including infiltration of the mucosa and epithelium with activated T cells, mast cells, and eosinophils. T cells and mast cells release cytokines that promote eosinophil growth and maturation and the production of IgE antibodies, and these, in turn, increase microvascular permeability, disrupt the epithelium, and stimulate neural reflexes and mucus-secreting glands. The result is airways hyperreactivity, bronchoconstriction, and hypersecretion, manifested by wheezing, coughing, and dyspnea.
Traditionally, asthma has been treated with oral and inhaled bronchodilators. These agents help the symptoms of asthma, but do nothing for the underlying inflammation. Recognition during the last 10 years of the importance of inflammation in the etiology of asthma has led to the increased use of corticosteroids, but many patients continue to suffer from uncontrolled asthma.
Scientists have determined that the leukotrienes (of which there are A, B, C, D, and E subtypes) plays a crucial role in asthma. They cause airways smooth muscle spasm, increased vascular permeability, edema, enhanced mucus production, reduced mucociliary transport, and leukocyte chemotaxis.
Like related prostaglandin compounds, leukotrienes are synthesized from arachidonic acid in the cell membrane. Arachidonic acid in mast cells, eosinophils, macrophages, monocytes, and basophils is formed from membrane phospholipids by the activation of phospholipase A2. After its formation, arachidonic acid undergoes metabolism via two major pathways: the cyclooxygenase pathway (which produced various prostaglandins and thromboxanes) and the 5-lipoxygenase pathway (which produces leukotrienes). A schematic of arachidonic acid metabolism is illustrated in FIG. 4. The prostaglandins, thromboxanes, and leukotrienes are known collectively as eicosanoids.
Anti-leukotrienes are members of a heterogeneous class of anti-asthma agents with the potential to interfere with the initial steps in the inflammatory cascade. Leukotrienes are inflammatory substances related to prostaglandins; both are generated from arachidonic acid in cell membranes. After arachidonic acid in mast cells, eosinophils, macrophages, monocytes, and basophils is formed, it is metabolized via two major pathways: (1) a cycloxygenase pathway (which produces prostaglandins and thromboxanes) and (2) the 5-lipoxygenase pathway, which produces leukotrienes in the cytoplasma. The leukotrienes are well known in medical science as the slow reacting substance of anaphylaxis (“SRS-A”). Leukotrienes play an important role in bronchial inflammation. They induce migration, adhesion and aggregation of various white blood cells (e.g., neutrophils, eosinophils, and monocytes) to blood vessels, increase capillary permeability, and cause bronchial and vessel smooth muscle constriction. The results include interstitial edema, leukocyte chemotaxis, mucus production, mucociliary dysfunction, and bronchospasm in the lungs. Certain classes of leukotrienes, for example, the cysteinyl leukotrienes (LTD4), are particularly potent bronchoconstrictors, being approximately 100 to 1,000 times more active than histamine. Leukotrienes, including cysteinyl leukotrienes, are released from mast cells during degranulation.
A number of anti-leukotrienes that either block leukotriene receptors or prevent leukotriene synthesis by blocking the enzyme 5-lipoxygenase are under investigation and in commercial use. The leukotriene inhibitors are heterogeneous in action: some block 5-lipoxygenase directly, some inhibit the protein activating 5-lipoxygenase, and some displace arachidonate from its binding site on the protein. The leukotriene antagonists, by contrast, block the receptors themselves that mediate airways hyperactivity, bronchoconstriction, and hypersecretion.
Human lung mast cells produce tumor necrosis factor (TNF), IL-4 and IL-5 after IgE stimulation in vitro (Chest 1997; 1 12:523-29). Immunohistochemical analysis in endobronchial biopsy specimens has confirmed this together with IL-6 production. Further, mast cell counts and TNF are statistically more significant in asthmatics when compared to normal subjects. TNF and IL-4 can potentiate up-regulation of the expression of vascular cell adhesion molecule-1 (VCAM-1)—an adhesion molecule of the immunoglobin super family—in the endothelial layer of the bronchial vasculature. Eosinophils, basophils and mononuclear cells display the very late activation antigen 4 (VLA-4) integrin on their cellular surfaces, which interacts with VCAM-1. Thus, through the interaction VLA-4/VCAM-1, TNF and IL-4 facilitate the recruitment of circulating leukocytes. The capacity of mast cells to release preformed cytokines (TNF) on IgE-mediated stimulus or to rapidly synthesize others (IL-4, IL-5) could be the initial event leading to bronchial inflammation. In fact, the induction and activation of TH2 clones, through a further production of cytokines, facilitates the activation and recruitment of the eosinophils, which act as the terminal effectors of the inflammatory reaction. In turn, the cytokines produced by leukocytes (TH2 cells, in particular) profoundly affect the development, activation, and priming of mucosal mast cells, thus promoting a positive proinflammatory loop. The recent findings that human mast cells produce IL-8 and that murine pulmonary-derived mast cells express both chemokines, monocyte chemoattractant protein-1 and macrophage inflammatory protein-1. This suggests that, besides the cytokines classically involved in leukocyte recruitment (IL-4, IL-5, TNF), mast cells also elaborate additional, potent chemoattractants in the airways, acting on eosinophils and polymorphonuclear leukocytes (IL-8). Moreover, because chemokines acting as histamine-releasing factors elicit mast cell degranulation, they may further sustain an autocrine activating loop.
The mast cells also play a key role in B-cell growth to provide the cell contact (like basophils) that is required, along with IL-4, for IgE synthesis in vitro, which suggests that mast cells may directly regulate the production of IgE independently of T-cells, and may, upon IgE cross-linking, generate a sufficient amount of IL-4 to initiate a local TH2 response, the subset of T-cells considered to play a central role in atopic asthma. Moreover, mast cells can also act as an antigen-presenting cell to T-lymphocytes, suggesting an even larger role for mast cells in the immune network of asthma.
Inhibition of mast cell degranulation by N-formyl-methionyl-leucyl—phenylalanine was reported in Inflammation, Vol. 5, No. 1, pp. 13-16 (1981). There, it was reported that two structurally different chemotactic peptides, i.e., pepstatin and N-formyl-methionyl-leucyl-phenylalanine, inhibit the increase in vascular permeability produced by intradermal injection of 40/80, anti-rat IgE serum, or macromolecular anionic permeability factor isolated from calf lung in rat skin. It also has been reported that these peptides appear to act directly on the mast cells.
Because of the importance of treating inflammatory diseases in humans, particularly, for example, asthma, arthritis and anaphylaxis, new bioactive compounds having fewer side effects are continually being sought. The inhibition of mast cell degranulation by the intervention of novel peptides of the present invention within the context of the asthma inflammatory process is visually depicted in FIG. 4.